The present invention relates to track-following a servo track during writing operations in a longitudinal tape system, and more particularly, to recovering from a stop write state after track-following is interrupted.
Current longitudinal tape drives, such as IBM® LTO Generation 5 tape drives, IBM® Jaguar 4 tape drives and later, among others, have methods to detect if the track following servo is getting off track, for whatever reason, and then shuts down the writing and track following activities to prevent the drive from over writing data in adjacent tracks. The shutdown is performed by monitoring a position error signal (PES) and detecting when the PES value exceeds a threshold value, commonly referred to as a “stop write” limit, for obvious reasons. If the PES value ever exceeds the stop write limit, the tape drive halts the write operation and stops track-following the servo track. When the PES value moves into an acceptable range, e.g., a range where the PES values are below the stop write limit by a certain amount, the drive re-acquires track-following lock and allows write operations to continue. This process stopping track-following and then later to recover from the stop write state and re-acquire track-following may take some time. And with the new flangeless tape paths of current tape drives, the process of re-acquiring track-following may be problematic and may require additional time to re-acquire the track-following state.
During this time when the tape drive is in the stop write state, the tape is still moving but data is not being written to tape, since the tape drive is in the stop write state. This, obviously, consumes tape storage capacity corresponding to the length of tape that passes by the head while the tape drive is in the stop write state.